The field of the invention is semiconductor fabrication. The invention concerns production of porous III-V semiconductors having tunable light emitting properties.
Group III-V materials possess light emitting properties, the properties of which are determined by the electronic band structure. In turn, the band structure is a function of the elemental composition of the material. A great deal of effort has been devoted to developing processed forms of III-V semiconductors with tunable light emission properties, e.g., quantum wells, wires and dots, superlattices, and bandgap engineered materials. Silicon, in its naturally occurring elemental form, is not light emitting. Silicon may be changed to porous silicon, a modified form of silicon. The unique electronic, morphological, and thermal properties of porous silicon make it useful for a range of applications. Porous silicon may even be light emitting, making it useful in optoelectronics. The successful development of porous silicon suggests that if III-V semiconductors could be made porous efficiently they might also display interesting characteristics.
In addition to potential applications in silicon-based optoelectronics, porous silicon has been used as an antireflective coating for silicon solar cells. Chemically modified porous silicon may be useful in chemical and biochemical sensing. Porous silicon can serve as an efficient matrix for direct introduction of high mass biomacromolecules in mass spectrometry. In sum, porous silicon is useful in numerous applications and is likely to find many additional uses in the future.
Conventional methods for producing porous silicon are often time-consuming, difficult, or ineffective in producing stable porous silicon structures. Equipment such as a potentiostats and illuminating light sources are required in etching processes of conventional porous silicon production methods. Porous silicon is normally produced by anodic etching, with (n-type) or without (p-type) illumination. In the anodic etch process mobile holes are electrically driven to the silicon-electrolyte interface where they participate in the oxidative dissolution of surface silicon atoms. Spatial anisotropy results from the potential barrier developed at the sharp tips of the evolving structures, which blocks further hole transport thus preventing further etching and giving rise to the porous structure. Porous silicon has also been made without external bias by chemical etching in HNO3/HF solutions (stain etching), and by photochemical etching.
Stain etching is typically slow (characterized by an induction period), inconsistent in result, unreliable in producing light-emitting porous silicon and is not readily amenable to lateral patterning. Stain etching is mainly used for making very thin porous silicon layers. Recently, it was shown that evaporating and annealing 150-200 nm of aluminum (Al) on Si results in more rapid stain etching. However, the porous silicon produced by this aluminum enhanced stain etching was approximately ten times weaker in luminescence than anodically etched porous silicon of a similar thickness, and the process still exhibits an induction period prior to commencement of etching. See, D. Dimova Malinovska et al., xe2x80x9cThin Solid Filmsxe2x80x9d, 297, 9-12 (1997). It has also been reported that Pt could be deposited electrochemically from a Pt (IV) solution onto Si during etching to produce light-emitting porous silicon, although it proved difficult to control the applied potential to affect both silicon etching and Pt deposition simultaneously. See, P. Gorostiza, R. Diaz, M. A. Kulandainathan, F. Sanz, and J. R. Morante, J. Electroanal. Chem. 469, 48 (1999).
Group III-V materials have characteristic emission bands. For nearly a decade, GaN-based materials have been the subject of intensive research for optoelectronics applications in the short-wavelength range. Despite the lack of bulk GaN substrates, GaN-based devices have been made possible by advances in materials growth. Research targeting GaN and related ternary materials and development of devices based on these materials have intensified in the past few years due to the evident value of short-wavelength optical devices and high power electronic devices based on this system. Because bulk GaN substrates are not available for epitaxial growth, GaN is most often grown on sapphire or SiC. Growth of high-quality GaN-based materials on these substrates must overcome the limitations posed by the large mismatch in lattice constants (e.g., 14% for sapphire) and thermal coefficients, so material quality has been studied extensively by both structural and optical techniques.
Because the interest in GaN centers around its short-wavelength light emission properties, it would be of great technological interest if the emission could be moved to even shorter wavelengths ( less than 368 nm), or equivalently higher energies, than that observed from bulk epitaxial GaN.
The present method produces porous Group III-V material with tunable morphologies and light emitting properties. In the method of the invention, a thin discontinuous metal layer is deposited on a Group III-V material surface. Preferred metals are Pt and Au. Au tends to produce a smoother morphology, while Pt tends to shift the Group III-V conduction band further toward the blue region. It is important that the deposited layer be sufficiently thin that it forms a discontinuous film, thereby providing access of etchant species to the Group III-V material surface in the area of the deposited metal. The surface is then etched in a solution including HF and an oxidant for a brief period, as little as 2 seconds to as much as 60 minutes. A preferred oxidant is H2O2. Morphology and light emitting properties of porous Group III-V material can be selectively controlled as a function of the type of metal deposited, doping type, thickness of metal deposit, doping level, whether emission is collected on or off the metal coated areas, and etch time. It is important that etching occurs both directly under the metal-coated area and in areas adjacent to the metal coated areas. Electrical assistance is unnecessary during the chemical etching of the invention.